1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor layer having a crystalline structure formed on a substrate having an insulation surface, and in particular a method for manufacturing a semiconductor device in which the corresponding semiconductor layer is used in an active layer. Especially, the invention is oriented at a method for producing a thin film transistor in which an active layer is formed from a crystalline semiconductor layer. Also, in the specification, a xe2x80x9csemiconductor devicexe2x80x9d includes all types of devices which can function by utilizing semiconductor characteristics, wherein the category thereof includes electro-optical devices represented by an active matrix type liquid crystal display device formed by using a thin film transistor, and an electronic apparatus in which such an electro-optical device is incorporated.
2. Background of the Invention
A thin film transistor (hereinafter called xe2x80x9cTFTxe2x80x9d) has been developed, in which an amorphous semiconductor layer is formed on an insulation substrate having a light transmission property such as glass, and a crystalline semiconductor layer which is crystallized by a laser annealing method and a thermal annealing method, etc. In many cases, glass substrates made of barium borosilicate glass and aluminum borosilicate glass, etc., are used. Since the market price of such glass substrates are inexpensive, though they are inferior to quartz substrates in terms of heat-resistance, such glass substrates have an advantage in that large-area substrates can be easily produced.
The laser annealing method has been known as a crystallizing technique by which amorphous semiconductor layers can be crystallized by providing a high energy onto only the amorphous semiconductor layers without raising the temperature of the glass substrates. In particular, it is considered that an excimer laser which is capable of providing a large output of short wavelengths is optimal. The laser annealing using an excimer laser is carried out by processing the laser beam so as to, by an optical system, make the beam spot-like or linear on a surface to be irradiated, and by scanning with the processed laser beam on the irradiated surface (relatively moving the irradiation position of the laser beam with respect to the irradiated surface). For example, an excimer laser annealing method in which a linear laser beam is used enables laser annealing on the entire irradiated surface by scanning in a direction orthogonal to the lengthwise direction thereof. Since the productivity thereof is excellent, the laser annealing method becomes the main stream as the production technology of a liquid crystal display device using TFT. The technology enabled a monolithic type liquid crystal display device in which a pixel TFT forming a pixel portion and a TFT of a drive circuit secured around the pixel portion are formed on a single glass substrate.
However, the crystalline semiconductor layer produced by the laser annealing method is formed by gathering a plurality of crystalline grains, and the position and size of the crystalline grains are random. TFTs produced on the glass substrate are formed so that the crystalline semiconductor layer is separated into island-like patterns for the separation of elements. In this case, the TFT could not be formed while specifying the positions and sizes of the crystalline grains. On the boundary of crystalline grains (crystalline grain phase), a lowering of the current carrying characteristics of a carrier was caused by the influences of a potential level at the re-coupling center, capturing center and crystalline grain boundary resulting from the amorphous structure and crystallizing defect. But, it is almost impossible that a channel forming region in which the crystalline characteristics seriously influence the characteristics of the TFTs is formed with mono-crystalline grains by excluding the influences of the crystalline grain boundary. Therefore, the TFTs, in which crystalline silicon film is used an active layer, having characteristics equivalent to those of a MOS transistor produced on a mono-crystalline silicon substrate could not be obtained until today.
In order to solve these problems, an attempt to increase the crystalline grain size has been made. For example, in xe2x80x9cHigh-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Methodxe2x80x9d, prepared by K. Shimizu, 0. Sugiura and M. Matsumura, IEEE Transactions on Electron Devices, Vol. 40., No. 1, pp112-117, 1993, a report is given of a dual beam laser annealing method in which a film of three layered (Si/SiO2/Si) structure is formed, and an excimer laser beam is irradiated from both the film side and the substrate side. According to the method, by irradiating a laser beam at a specified energy intensity, it is possible to increase the crystalline grain size.
In a monolithic type liquid crystal display device, a pixel portion for performing image display and a drive circuit are formed on the same substrate. The pixel portion is provided with a pixel TFT and a holding capacitor while the drive circuit is provided with a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, etc., which are formed on the basis of a CMOS circuit. However, the operating conditions of the pixel TFTs are not the same as those of the drive circuit TFTs, whereby the characteristics requested in the TFTs differ to some degree. For example, the pixel TFT functions as a switching element, and is driven by applying voltage to liquid crystal. Since liquid crystal is driven by an alternate current, a system called xe2x80x9cframe reversing drivexe2x80x9d is frequently employed. In the system, the characteristics requested of the pixel TFTs is a sufficiently decreased OFF current value (that is, the drain current flowing when the TFTs are turned off) in order to suppress power consumption to a sufficiently low level. On the other hand, since a high drive voltage is applied to the buffer circuit of the control circuit, it is necessary that the withstand is sufficiently increased so that it is not broken when a high voltage is applied. Further, in order to increase the current drive capacity, it is necessary to sufficiently secure the ON current value (that is, the drain current flowing when the TFT is turned on).
Also, in order to control the threshold voltage (hereinafter called xe2x80x9cVthxe2x80x9d), which is an important characteristic parameter in the TFTs, in a specified range, it was necessary to decrease a charge defect population of the underground film, gate insulation layer and interlayer insulation film, which are formed of an insulation layer in close contact with the active layer, in addition to valence electron control of the channel forming region, and it also was necessary to take the balance of the internal stress into consideration. With respect to such a request, a material containing silicon such as a silicon oxide layer and a silicon oxide nitride layer, etc., as a component element was suitable.
Thus, in an attempt to improve the performance of a monolithic type liquid crystal display device, an attempt to improve the performance of TFTs by only increasing the crystalline grain size of a crystalline semiconductor layer forming the active layer is insufficient, and it was necessary to take various characteristics of the active layer, the underground film, gate insulation layer and interlayer insulation film, which are formed above and below the active layer, into consideration.
The present invention is a technology to solve these and other problems. It is therefore an object of the invention to achieve a semiconductor area formed of island-like patterns formed to be mono-crystalline or an area which can be regarded as monocrystal, and at the same time, to achieve a lamination structure which can stabilize various characteristics of the TFTs. Further, in a semiconductor device represented by a monolithic type liquid crystal display device in which a plurality of circuits are formed on the same substrate, wherein arrangement or orientation of TFTs having adequate properties is enabled on the basis of the specifications requested by the function circuits thereof, and the operation characteristics and reliability can be achieved remarkably well.
A laser annealing method is used as a method for forming a crystalline semiconductor layer from an amorphous semiconductor layer formed on a substrate such as glass. In the laser annealing method according to the invention, a pulse oscillation type or continuous light emission type excimer laser or argon laser is used as its light source, and a laser beam formed to be linear by an optical system is irradiated onto both the surface (in the specification, the surface is defined to be a plane on which an island-like semiconductor layer is formed) and rear side (in the specification, the rear side is defined to be a plane which is the opposite side of the plane where an island-like semiconductor layer is formed) of a substrate.
FIG. 3A is a view showing a configuration of such a laser annealing apparatus. The laser annealing apparatus has a stage 1202 on which a laser oscillator 1201, an optical system 1100 and a substrate are fixed. The stage 1202 is provided with a heater 1203 and a heater controller 1204, which can heat the substrate to a temperature of 100 through 450xc2x0 C. A reflection plate 1205 is provided on the stage 1202, and a substrate 1206 is installed thereon. In the construction of the laser annealing apparatus so composed as in FIG. 3A, a description is given of a holding method of the substrate 1206 with reference to FIG. 3B. The substrate 1206 held on the stage 1202 is installed in a reaction chamber 1213, and a laser beam is irradiated thereonto. The interior of the reaction chamber 1213 can be pressure-reduced or filled with an inactive gas by an exhaust system or a gas system, neither of which is illustrated, wherein a semiconductor can be heated to a temperature of 100 through 450xc2x0 C. without being contaminated. The stage 1202 can move in the reaction chamber along a guide rail 1216, wherein a linear laser beam can be irradiated onto the entire surface of the substrate. The laser beam comes in through a quartz window (not illustrated) secured on the upper surface of the substrate 1206. Also, in FIG. 3B, a transfer chamber 1210, an intermediate chamber 1211, and a load/unload chamber 1212 are connected to the reaction chamber 1213, which are separated by sluice valves 1217 and 1218. A cassette 1214 capable of holding a plurality of substrates is installed in the load/unload chamber 1212, and a substrate is transferred by a transfer robot 1215 secured in the transfer chamber 1210. The substrate 1206 indicates a substrate being transferred. With such a construction, the laser annealing can be continuously carried out in a pressure-reduced state or an inactive gas atmosphere.
FIG. 2A and B are views which explain an optical system construction of the laser annealing apparatus illustrated in FIG. 3A. An excimer laser and argon laser may be applied to the laser oscillator 1101. FIG. 2A is a view showing a state where the optical system 1100 is observed from the side, wherein the laser beam outgoing from the laser oscillator 1101 is split in the longitudinal direction by an cylindrical lens array 1102. The split laser beams are widened once they are collected by the cylindrical lens 1104, and are reflected by a mirror 1107. Thereafter, they are controlled by a cylindrical lens 1108 so that they become a linear laser beam on an irradiation plane 1109. Thereby, it is possible to make energy distribution of the linear laser beam in the width direction uniform. Also, FIG. 2B is a view showing a state where the optical system 1100 is observed from above. The laser beam outgoing from the laser oscillator 1101 is split in the cross direction by a cylindrical lens array 1102. After that, the laser beam is synthesized to be singular on the irradiation plane 1109 by a cylindrical lens 1105, whereby the energy distribution of the linear laser beam in the lengthwise direction can be made uniform.
Further, FIG. 1 is a view to explain the concept of a laser annealing method according to the present invention. An insulation layer 1002 is formed on a substrate 1001 such as glass, and an island-like semiconductor layer 1003 is formed thereon. The insulation layer 1002 may be an insulation layer whose constituents are a silicon oxide film, a silicon nitride film, a silicon oxide nitride film, and aluminum. That is, the insulation layer 1002 may be singular, or these films may be adequately combined with each other. And, a laser beam passed through a cylindrical lens 1005 having the same features as those of the cylindrical lens 1108 is irradiated onto the island-like semiconductor layer 1003 as a linear laser beam by the optical system 1100 described with reference to FIG. 2A and B. The island-like semiconductor layer 1003 is provided with a direct laser light components 1006 passed through the cylindrical lens 1005 and irradiated directly onto the island-like semiconductor layer 1003, and diffused laser components 1007 which penetrate the insulation layer 1002 and substrate 1001, are reflected by the reflection plate 1004, again penetrated the insulation plate 1001 and insulation layer 1002 and are irradiated onto the island-like semiconductor layer 1003. In either of the cases, since the laser beam that passed through the cylindrical lens 1005 will have an incident angle of 45 through 90xc2x0 with respect to the substrate surface in the process of condensing light, the laser beam reflected by the reflection plate 1004 is also reflected toward the inside direction of the island-like semiconductor layer 1003. At the reflection plate 1004, its reflection plane is formed of aluminum. If the reflection plane is mirror-finished, a positive reflectivity of approx. 90% can be obtained in a range of 240 through 320 nm in wavelength. Also, where the material is aluminum, and minute dents and projections of several hundreds of nanometers are formed, a diffusion reflectivity of 50 through 70% can be obtained (Integral reflectivityxe2x80x94Positive reflectivity).
Thus, a laser beam is irradiated onto both the surface and rear side of a substrate, and the island-like semiconductor layer 1003 formed on the substrate 1001 is laser-annealed on both sides thereof. In the laser annealing method, a semiconductor film is instantaneously heated and melted by optimizing the conditions of the laser beam to be irradiated, a generation density of crystalline nuclei and crystallizing growth from the crystalline nuclei can be controlled. Since the oscillation pulse width of the excimer laser is from several nanoseconds through several hundred nanoseconds, for example, 30 nanoseconds, if irradiation is performed with the pulse generation frequency set to 30 Hz, the semiconductor layer at the area where the laser beam is irradiated is instantaneously heated by a pulse laser beam, and it is cooled down for a considerably longer period of time than the heating time.
Irradiation of a laser beam onto only one side heats only one side of the semiconductor layer formed on a substrate, the cycle of heating to melt and solidification to cool becomes very rapid, and sufficient crystallizing growth cannot be expected even though the generation density of crystalline nuclei can be controlled. However, if a laser beam is irradiated onto to both sides of a semiconductor layer, the cycle from heating to melt to solidifying by cooling becomes gentle, whereby the time permitted for nuclei growth in the process of cooling and solidification becomes relatively longer, and sufficient crystallization growth can be obtained.
In the wavelength of an excimer laser beam, the laser beam can be absorbed into only the extreme top surface of a semiconductor layer, where the laser beam is converted to heat. For example, in the case of an XeCl excimer laser beam having a wavelength of 308 nm, almost all the laser beam can absorbed in an area 20 nm deep from the extreme top surface of a silicon layer. After that, the laser beam is thermally transmitted from this area to deeper inner silicon layers, whereby the entire silicon layer is annealed. That is, while the laser beam is being irradiated, the surface temperature of the silicon layer always becomes higher in comparison with the other areas. This can be easily presumed on the basis of the results of a thermal transmission simulation in laser annealing.
Herein, it is assumed that the energy, which is absorbed into a silicon layer and is thermally converted, in the case where a laser beam is irradiated from the surface of a substrate onto one side thereof is the same as that in the case where the laser beam is irradiated onto both the surface and the rear side thereof. FIG. 26 shows the results of simulations of the laser beam intensity distribution for one-sided irradiation and both-side irradiation in the depth direction of a silicon layer. In the case of both-side irradiation, a case is indicated where the ratio of the surface irradiation intensity to the rear side irradiation intensity is 3 to 1. As shown in FIG. 26, in the process of rising the temperature by irradiation by a laser beam, the areas that absorbs the laser beam and generates heat in the case of both-side irradiation will be two planes which are the surface side and the underground boundary side. That is, it is possible to effectively expand the heat-generating area. Therefore, in comparison with one-sided irradiation, abrasion will barely occur (it has been known that abrasion occurs if the laser energy density exceeds a specified level where an excimer laser beam is irradiated onto a semiconductor layer). That is, in the case of both-sided irradiation, it is possible to heat the semiconductor layer at an effectively high energy density without generating any abrasion on the semiconductor layer.
With the present invention, an island-like semiconductor layer can be formed as a mono-crystal area or an area which can be regarded as monocrystal by applying such a laser annealing method (dual beam laser annealing method), wherein it is possible to produce a semiconductor device having TFTs that have a structure responsive to the features or functions of respective circuits by using such an island-like semiconductor layer in the TFT active layer.
Therefore, in order to solve the abovementioned problems, a method for manufacturing a semiconductor device having a TFT provided on a substrate according to a first aspect of the invention is featured in comprising the steps of: forming an underground film in contact with one main surface of the substrate; forming an amorphous semiconductor layer on the underground film; forming an island-like semiconductor layer having a first shape from a semiconductor film having the amorphous semiconductor layer; irradiating a laser beam onto the surface opposed to the island-like semiconductor layer having the first shape of the substrate, and reflecting the laser beam, which comes from a peripheral area of the island-like semiconductor layer and has passed through the substrate, by a reflection plate secured at the side opposed to one main surface of the substrate, wherein an island-like semiconductor layer having a crystalline property is formed; removing a part overlapping at least a gate electrode or a part forming a channel forming region, of end parts of the island-like semiconductor layer having a crystalline property, by 1 xcexcm or more from the end parts, and forming an island-like semiconductor layer having a second shape; forming a high concentration n-type impurity region or a high concentration p-type impurity region which is made into at least a channel forming region and a source region or a drain region on the island-like semiconductor layer having the second shape; and doping hydrogen to the island-like semiconductor layer having the second shape; and irradiating the laser beam from the surface of the substrate side of the island-like semiconductor layer; and irradiating the laser beam from the surface of the substrate side of the island-like semiconductor layer; wherein at least the channel forming region of the TFT has a mono-crystalline structure, and to which hydrogen is doped.
A method for manufacturing a semiconductor device having a TFT provided on a substrate according to another aspect of the invention is featured in comprising the steps of: forming an underground film in contact with one main surface of the substrate; forming an amorphous semiconductor layer on the underground film; forming an island-like semiconductor layer having a first shape from a semiconductor film having the amorphous semiconductor layer; introducing a catalyst element, which fosters crystallization of a semiconductor, to the island-like semiconductor layer having the first shape; irradiating a laser beam onto the surface opposed to the island like semiconductor layer having the first shape of the substrate, and reflecting the laser beam, which comes from a peripheral area of the island-like semiconductor layer and has passed through the substrate, by a reflection plate secured at the side opposed to one main surface of the substrate, and irradiating the laser beam from the surface of the substrate side of the island-like semiconductor layer, wherein an island-like semiconductor layer having a crystalline property is formed; removing a part overlapping at least a gate electrode or a part forming a channel forming region, of end parts of the island-like semiconductor layer having a crystalline property, by 1 xcexcm or more from the end parts, and forming an island-like semiconductor layer having a second shape; forming a high concentration n-type impurity region or a high concentration p-type impurity region which is made into at least a channel forming region and a source region or a drain region on the island-like semiconductor layer having the second shape; and doping hydrogen to the island-like semiconductor layer having the second shape; wherein at least the channel forming region of the TFT has a mono-crystalline structure, and to which hydrogen is doped.
A method for manufacturing a semiconductor device having a TFT provided on a substrate according to still another aspect of the invention is featured in comprising the steps of: forming an underground film in contact with one main surface of the substrate; forming an amorphous semiconductor layer on the underground film; introducing a catalyst element, which fosters crystallization of a semiconductor, to the amorphous semiconductor layer; thermally processing the semiconductor layer having a crystalline property, and forming a semiconductor layer having a crystalline property; forming an island-like semiconductor layer having a first shape from a semiconductor film having the amorphous semiconductor layer; introducing a catalyst element, which fosters crystallization of a semiconductor, to the island-like semiconductor layer; irradiating a laser beam onto the surface opposed to the island-like semiconductor layer having the first shape of the substrate, and reflecting the laser beam, which comes from a peripheral area of the island-like semiconductor layer and has passed through the substrate, by a reflection plate secured at the side opposed to one main surface of the substrate, wherein an island-like semiconductor is crystallized; removing a part overlapping at least a gate electrode or a part forming a channel forming region, of end parts of the island-like semiconductor layer, by 1 xcexcm or more from the end parts, and forming an island-like semiconductor layer having a second shape; forming a high concentration n-type impurity region or a high concentration p-type impurity region which is made into at least a channel forming region and a source region or a drain region on the island-like semiconductor layer having the second shape; and doping hydrogen to the island-like semiconductor layer having the second shape; wherein at least the channel forming region of the TFT has a mono-crystalline structure, and to which hydrogen is doped.
The construction of the present invention is applicable to a method for manufacturing a semiconductor device having a pixel TFT secured in a pixel portion and a drive circuit, in which a p-channel type TFT and an n-channel type TFT are provided around the corresponding pixel portion, on the same substrate.